I. Field of the Invention
This invention relates to a unit for isolating a wellhead for removal and repair of the wellhead and, in particular, to a packer for a wellhead repair unit which securely packs off the well to permit safe removal of the wellhead and which subsequently can be retrieved mechanically to allow continued production.
II. Description of the Prior Art
Various devices have been developed to isolate a well for removal and repair of the wellhead in a geothermal well. The extreme temperatures and pressures found in geothermal wells makes it important to safely seal off the well prior to removal of the wellhead. The prior known repair systems utilize a hydraulically set packer which is dependent upon the pressure balance between the downhole pressure and the hydraulic pressure applied through the running tool from the surface. Some tools include means for increasing the gripping action within the casing in the event downhole pressure increases. To release the packer once the repairs are completed and the wellhead is replaced, the hydraulic pressure is increased to overcome the downhole pressure.
Because the past known repair units are dependent upon a critical pressure balance, variations in the downhole pressure can cause release and travel of the packer or in the extreme case a blowout of the tool. Pressure variations may also cause the hydraulically set packer to slip down into the casing. As a result, because the retrieval tool has limited downhole reach, re-engagement may not be possible requiring other fishing techniques or lost production. Alternatively, an increase in downhole pressure can cause the packer to form a harder grip with the casing wall requiring an increased hydraulic pressure to release the packer.
Other packers are well known for a variety of applications. However, not all packers are suitable for the environment of a geothermal well. Moreover, many packers are not retrievable and are merely drilled out when further work is needed on the well. These are not suitable alternatives for a completed and operated geothermal well.